The present invention relates to methods and apparatus for providing variable harmonic distortion in an audio circuit, such as an audio amplifier.
Designers of guitar amplifiers back in the 1950's focused on creating amplifiers with very little distortion. This was not an easy task as solid state devices were not generally available and most amplifiers were designed and built using vacuum tubes. Although tube amplifiers inherently have more distortion than solid state amplifiers, the early amplifier pioneers tried in vain to produce amplifiers with little or no distortion. Most of their designs had a high order of distortion, especially second harmonic distortion.
In the context of electric guitar amplification, the existence of harmonic distortion in the amplifier is not undesirable. Indeed, it has been generally thought that even order harmonics are pleasing to the ear, (especially the second harmonic), while odd order harmonics are thought to sound “raucous,” “raspy,” and unpleasant, especially the third and fifth harmonics. The desirability of even order harmonics appears to hold even in fairly high ratios to the normal (undistorted) signal.
Thus, guitar amplification has become a niche technology within the broader universe of the audio amplification arts. What sets a guitar amplifier apart from an amplifier found in, for example, a sound system or a home stereo is that the latter seeks to reproduce sounds as closely as possible to the input signal, while a guitar amplifier significantly alters the sound and is, in essence, an extension of the instrument and a part of an overall guitar system.
Most rock guitar players use solid body guitars, which provide some degree of resonance but not nearly as much as hollow body guitars or acoustic guitars. A solid body guitar is preferred because the body is formed of a solid piece of material and, therefore, is not as prone to extraneous vibrations in response to the sound pressure waves from the amplification system. Such extraneous vibrations are also known as acoustic feedback and are to be avoided. A solid body guitar is basically a sine wave generator in that it does not produce much frequency content beyond the frequencies of the resonating guitar strings. Thus, most of the harmonics associated with the characteristic sounds identifiable as an electric guitar actually come from the amplifier and not the guitar. The harmonics introduced by the guitar amplifier differentiate one guitar system (e.g., guitar and amplifier combination) from another, thereby giving each system its own “personality.”
The inherent tendency for vacuum tube amplifiers to produce harmonic distortion has made them a favorite among electric guitar players and there have been numerous design approaches to simulate vacuum tube designs in solid state amplifiers. Indeed, various kinds of asymmetrical clippers and various kinds of distortion producing circuitry have been produced to simulate second harmonic distortion. Because tube amplifiers have been around for so long, most contemporary tube amp designs utilize very similar circuitry. Most high-power tube type guitar amplifiers utilize push-pull output stages operating in class A/B. The A/B class dictates that the output signal is shared by two or more output tubes which are driven by signals that are equal but 180 degrees out of phase. Conventional wisdom dictates that the drive signals to the output tubes (specifically the grids) should be balanced to minimize distortion and limit output transformer core saturation problems (caused by asymmetrical currents through the primary).
While the design of an amplifier in accord with the conventional wisdom may be acceptable to users seeking amplifiers that minimize distortion, a guitar player will likely find the lack of harmonic content objectionable. Further, even if a guitar amplifier (whether of the tube type or solid state) introduces harmonic distortion, it would be desirable to make such distortion variable so that the player may tune the sound to his or her liking.